How to find/select a security code auditor?

We have an internal library with some reusable functionality, including auth. I'm looking to get some external validation of the code from a security perspective.

Is there some reputable list/site/accreditation/member's directory I should know about?

I've not had much luck googling - or rather, trusting the efficacy of the results beyond identifying the consultants/companies with the craftiest SEO.

TIA